Cambridge A Level Chemistry 9701 — 2010 Oct/Nov Paper 5 · Variant 1

This document consists of 9 printed pages and 3 blank pages. DC (NF/DJ) 30596/3 © UCLES 2010 [Turn over UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certificate of Education Advanced Level * 9 8 0 0 4 5 4 6 4 4 * CHEMISTRY 9701/51 Paper 5 Planning, Analysis and Evaluation October/November 2010 1 hour 15 minutes Candidates answer on the Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs, or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions. You are advised to show all working in calculations. Use of Data Booklet is unnecessary. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 2 Total

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3 9701/51/O/N/10 © UCLES 2010 [Turn over For Examiner’s Use 1 When aqueous sodium chloride, NaCl, is added to aqueous lead nitrate, Pb(NO3)2, a white precipitate of lead chloride, PbCl 2, is produced. A suggested stoichiometric equation is Pb(NO3)2(aq) + 2NaCl(aq) PbCl2(s) + 2NaNO3(aq) In separate experiments, different volumes of 0.20 mol dm–3 aqueous sodium chloride are added to a fixed volume of 0.10 mol dm–3 aqueous lead nitrate. In each case, the precipitate is filtered, washed with distilled water and thoroughly dried. The mass of the precipitate is recorded. You are to plan an experiment to investigate this reaction in order to confirm or reject the stoichiometry of the equation. (a) By considering the suggested stoichiometric equation, predict and explain how the number of moles of the precipitate, PbCl 2, will change as the number of moles of NaCl added increases. Prediction … … … Explanation … … … [2] (b) State a limiting factor that must be taken into account when increasing the volume of the aqueous sodium chloride added. … Sketch the graph which would result if, after some of the experiments, the NaCl is in excess. Start your graph with no NaCl added. PbCl2 / mol NaCl / mol 0 0 [3]

4 9701/51/O/N/10 © UCLES 2010 For Examiner’s Use (c) In the experiment you are about to plan, identify the following. (i) the independent variable … (ii) the dependent variable … (iii) another variable to be controlled … [2] (d) Design a laboratory experiment to test your prediction in (a). You are provided with 250 cm3 of 0.20 mol dm–3 aqueous sodium chloride. (i) Outline how you would prepare 250 cm3 of 0.10 mol dm–3 aqueous lead nitrate. [Ar: N, 14; O, 16; Pb, 207] (ii) Give a step by step description of how you would carry out one experiment. You should state the volumes of each solution to be used, • how the volumes will be measured, • how you would dry the precipitate. • … … … … … … … … … … … … … … … … …

5 9701/51/O/N/10 © UCLES 2010 [Turn over For Examiner’s Use … … … …[6] (e) In the table below enter appropriate headings to show additional data you would record when carrying • out your experiments and the values you would calculate in order to construct a graph to support or reject your prediction in (a). The headings should include the appropriate units, enter the volumes from your plan in • (d), enter suitable volumes for four further experiments. • [2] (f) How would you ensure that at the end of each experiment the precipitate was thoroughly dried? … … [1] [Total: 16]

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7 9701/51/O/N/10 © UCLES 2010 [Turn over For Examiner’s Use 2 The melting point of solid water is 0 °C. This is the same as the freezing point of water. This freezing point can be lowered (depressed) by the addition of a solute, such as glucose. The extent of the freezing point depression depends on the number of particles of solute dissolved in the solution. The freezing point depression, ΔTf, is proportional to the molal concentration, cm, of the solution. ΔTf = Kf cm where Kf is the freezing point depression constant. The molal concentration (molality) of a solution is defined as the number of moles of a solute dissolved in one kilogram of water e.g. a one molal solution has one mole of solute dissolved in one kilogram of water. An experiment was carried out to investigate the relationship between ΔTf and cm. A weighed sample of distilled water was placed in a boiling tube. • A weighed sample of glucose was added. • The mixture was stirred until a solution was obtained. • The tube was placed in a freezing apparatus to lower the temperature. • The freezing point of the solution was measured precisely and the freezing point • depression calculated.

8 9701/51/O/N/10 © UCLES 2010 For Examiner’s Use (a) Calculate the Mr of glucose C6H12O6. [Ar: H, 1.0; C, 12.0; O, 16.0] [1] (b) The results of the experiment are recorded below. A B C D E F mass of water / g mass of glucose / g freezing point depression ΔTf / °C 100 10.0 1.03 100 12.2 1.26 100 18.0 2.09 100 23.3 2.40 100 27.7 2.86 100 30.9 3.22 100 33.1 3.31 100 38.6 3.98 100 42.3 4.37 Process the results in the table to calculate the molality of the glucose solution. This will enable you to plot a graph to show how the freezing point depression, ΔTf, varies with the molality of the solution. Record these values to three significant figures in the additional columns of the table. You may use some or all of the columns. Label the columns you use. For each column you use include units where appropriate and an expression to show how your values are calculated. You may use the column headings A to F for this purpose. [2]

9 9701/51/O/N/10 © UCLES 2010 [Turn over (c) Present the data calculated in (b) in graphical form. Draw the line of best fit. [3]

10 9701/51/O/N/10 © UCLES 2010 For Examiner’s Use (d) Circle on the graph any point(s) you consider to be anomalous. For any point circled on the graph suggest an error in the conduct of the experiment that might have led to this anomalous result. … … … … … … … … … [3] (e) (i) Determine the value of ΔTf / cm from your graph. This is the freezing point depression constant K f . Mark clearly on the graph any construction lines and show clearly in your calculation how the intercepts were used in the calculation of the slope. (ii) By considering the data you have processed and the graph you have drawn, decide if the experimental procedure described is suitable for the determination of the freezing point depression constant Kf. Explain your reasoning. [3]

11 9701/51/O/N/10 © UCLES 2010 For Examiner’s Use (f) When the experiment was repeated using sodium chloride instead of glucose as the solute, the freezing point depressions were found to be twice the value obtained in the glucose experiment for each molality. Using the information given at the start of the question suggest a reason for this. … … … [1] (g) Using your suggestion from (f) predict the effect on the freezing point depression if a weak acid such as ethanoic acid was used instead of glucose or sodium chloride as the solute. … … [1] [Total: 14]

12 9701/51/O/N/10 © UCLES 2010 Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. University of Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge. BLANK PAGE

UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the October/November 2010 question paper for the guidance of teachers 9701 CHEMISTRY 9701/51 Paper 5 (Planning, Analysis and Evaluation), maximum raw mark 30 This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes must be read in conjunction with the question papers and the report on the examination. • CIE will not enter into discussions or correspondence in connection with these mark schemes. CIE is publishing the mark schemes for the October/November 2010 question papers for most IGCSE, GCE Advanced Level and Advanced Subsidiary Level syllabuses and some Ordinary Level syllabuses.

Page 2 Mark Scheme: Teachers’ version Syllabus Paper GCE A LEVEL – October/November 2010 9701 51 © UCLES 2010 Question Sections Indicative material Mark 1 (a) PLAN Problem Predicts a direct proportionality. Accept statements such as ‘no. of moles of the precipitate/PbCl2 will increase (as the number of moles of NaCl increases’). Equation shows a 1 to 2 molar ratio or wtte. (If a ‘plateau’ graph is described for the first mark allow a correctly explanation for the second mark i.e. all the lead nitrate has been used up) [1] [1] (b) PLAN Method & Problem & ACE All the lead nitrate was used up; moles or concentration of lead nitrate; total volume of lead nitrate. NOT amount A diagonal straight-line going through the origin. The line will abruptly change to a horizontal line Possible alternatives: • Diagonal line only, with +ve slope from the origin – 1 mark • Diagonal line not starting at the origin with a horizontal line – 1 mark • Curve from the origin with decreasing gradient and horizontal straight-line – 1 mark • Curve not from the origin with decreasing gradient and horizontal straight-line – 0 marks • Any lines showing an increase in gradient – 0 marks [1] [1] [1] (c) PLAN Problem Independent variable – volume/mass/moles of NaCl Dependent variable – moles/mass of PbCl2 / ppt Other variables – temperature. NOT amount and NOT concentration of the NaCl. Three points correct – 2 marks Two points correct – 1 mark Any incorrect suggestions cancel correct suggestions [2]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE A LEVEL – October/November 2010 9701 51 © UCLES 2010 Question Sections Indicative material Mark (d) (i) (ii) PLAN Method Calculating an appropriate mass of lead nitrate; this should be 8.275(8.3)g Dissolving the solid in water (or stirring) in a beaker or other appropriate vessel (volumetric/graduated/dilution flask) using less than 250 cm3 Adding to the volumetric/graduated/dilution flask and adding water to the 250 cm3 mark (if 250 cm3 of water are added directly to a volumetric/ graduated/dilution flask (containing the solid of course) allow 1 mark) Any dilution from a ‘given’ solution of lead nitrate gets 0 marks out of 3. Synthesis of the lead nitrate from lead and nitric acid scores zero. A student who uses 33.1 grams of lead nitrate to make up correctly 1 dm3 of solution can gain the first two marks, but to gain the third mark a measured 250 cm3 needs to be taken using an appropriate measuring vessel. Apparatus for volume measurement (burette/pipette/measuring cylinder)(not a syringe) used to measure 50 cm3 or less of lead nitrate and 100 cm3 or less of sodium chloride (mention of only one measuring vessel is enough for this mark) Method for drying the precipitate (adding propanone and allowing to evaporate/pressing with filter paper/warm oven/sun leaving out to dry. NOT heat or the use of a Bunsen or microwave. [1] [1] [1] [1] [1] (e) PLAN Method Table needs • volumes of lead nitrate, sodium chloride and mass/weight of lead chloride/ppt with appropriate data (volumes must total no more than 250 cm3 in each case) • moles of sodium chloride and lead chloride/ppt • correct units throughout (/cm3, /cm3 and /g [accept grams]; accept ( ) instead of / ) (these are for the items in bullet point one) Normally there will be 5 sets of volumes with the volume of lead nitrate being constant. (no figures are required for the mass of the ppt) (allow 4 sets if the volumes are different to those in (d)). Ignore; volumes of water; units in the mole columns; items such as filter paper; numbers in the mole columns. Do not allow; n for the number of moles; zero for the volume of lead nitrate or sodium chloride. [1] [1] [1] (f) PLAN Method The drying process should be repeated to constant mass Allow heat/reheat to constant mass/weight. [1] Total [16]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE A LEVEL – October/November 2010 9701 51 © UCLES 2010 Question Sections Indicative material Mark 2 (a) ACE Data The required Mr is correct (180) (allow if given in the table.) Ignore units. [1] (b) ACE Data The required two column headings are correct. • Moles of glucose/solute; B/180; B/Mr; mass of glucose/Mr etc. IGNORE units. • Molality; D × 10; D/(A × 10–3) or equivalent; units mol kg–1 (allow Cm instead of molality) The calculations are correct and both columns are fully completed to 3 s.f. (allow ecf for the calculations, only if the calculation is absolutely clear) If only one column is given with correct header and numbers give one mark. If the molality column is given and it is fully correct give both marks (the formula in the heading needs to be something like (B × D × 10)/180 or D × 10 if the D column heading is correct as B/180 / or equivalent), if only the heading or the numbers are correct give one mark (even if column D is incorrect in some way, this means that a correct set of values for the molality, to 3sf could be worth a mark). If the first two values for the molality are 0.555 and 0.677 and the remainder are all correct award the mark. Table values at the end. [1] [1] (c) ACE Data Give one mark for labelling the x-axis molality and the y-axis freezing point depression provided the plotted points and the origin if used cover at least half the scalings in both directions. Scales must allow all the points to be plotted; if less than nine points are plotted allow plotting for those actually plotted. (in other cases where correct scales are used all 9 points must be plotted, allow plotting to the nearest half a square in each direction) (If either of the axes have non-linear scales do not award the first two marks) Freezing point depression must have units /°C; ignore any molality units Give one mark for correctly plotting all the points plotted. Give one mark for drawing a ‘line of best fit’ passing through the origin. (again allow a half square accuracy) (If the origin is not used do not give this mark) These three points can stand alone as far as possible while allowing for ecf. Even if the axes are unlikely to access the mark, allow the other two marks. [1] [1] [1]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE A LEVEL – October/November 2010 9701 51 © UCLES 2010 Question Sections Indicative material Mark (d) ACE Data One mark if the two anomalous points furthest from the line (one on each side) are circled. Allow only one anomaly if there is only one or all the anomalies are on the same side. In plotting the points, it is possible that some points will be a little way from the correctly drawn line. These in many cases are likely not to be ‘ringed’. If 5 or more points are ‘ringed’ do not award this mark but allow any subsequent correct discussion. For each of the two different anomalies an appropriate explanation gains one mark. If the graph is plotted correctly point 3 will be above the line and point 7 below the line. POINT 3; temperature measured after freezing was complete (i.e. late); too much glucose added; reduced amount of water. POINT 7; temperature recorded before freezing was complete; not all glucose dissolved; too much water. If two correct suggestions are given but not ‘tied’ to a particular point award one mark. If the comments are assigned to the wrong points NO marks. [1] [2] (e) (i) (ii) ACE data Appropriately drawn lines on the graph with correctly deduced intercepts. For a correct calculation from the candidates figures with correct units (°C kg mol–1) Since the results produce a good linear graph, the procedure is OK. Accept ‘constant gradient’, and references such as ‘most of the points are close to the drawn line’ etc. ‘Line passing through the origin’ is irrelevant. [1] [1] [1] (f) ACE Data The NaCl produces twice the number of particles. Allow the statement that NaCl produces Na+ and Cl –. [1] (g) ACE Data Between answers to (a) and (f). [1] Total [14]

Page 6 Mark Scheme: Teachers’ version Syllabus Paper GCE A LEVEL – October/November 2010 9701 51 © UCLES 2010 A B C D E F mass of water /g mass of glucose /g freezing point depression ∆Tf /°C moles of glucose B 180 molality D × 10 mol/kg 100 10.0 1.03 0.0556 0.556 100 12.2 1.26 0.0678 0.678 100 18.0 2.09 0.100 1.00 100 23.3 2.40 0.129 1.29 100 27.7 2.86 0.154 1.54 100 30.9 3.22 0.172 1.72 100 33.1 3.31 0.184 1.84 100 38.6 3.98 0.214 2.14 100 42.3 4.37 0.235 2.35